The media mogul’s defense team will have one final chance on Wednesday to try to convince the Court of Cassation to throw out the conviction by a lower-level appeals court.

The tensely-awaited decision could have an impact on the fragile coalition of Premier Enrico Letta, since his center-left party depends on Berlusconi’s conservatives for survival.

Berlusconi’s conviction in a case of film rights sales involving his Mediaset media empire carried a four-year term in prison as well as the ban on the former three-time premier and current senator on holding any elected or appointed public office for the next five years.

If Berlusconi fails to have the guilty verdict trial overturned — or at least win a new trial — it will be the first definitive conviction after a raft of cases over the last two decades that either ended in acquittals or saw statutes of limitations expire.

Prosecutor Antonello Mura insisted the lower appellate court treated Berlusconi fairly. He cited several instances where the court in Milan allowed Berlusconi to legitimately skip hearings because of official duties such as Cabinet meetings.

Mura methodically hammered away at his points for more than four hours Tuesday afternoon, leaving many in the packed, stifling-hot courtroom slumped back in their seats and some on the five-judge panel checking their watches occasionally.

But he snagged attention when he ended by asking the court to reduce the ban’s duration from five to three years. Mura cited ambiguous interpretations of laws about such bans as an “accessory” punishment to accompany prison term.

One of Berlusconi’s lawyers, Franco Coppi, scoffed when asked if the request for a shorter ban was a kind of semi-victory for the defense.

Mura is just trying to rectify a `’blatant error” by the lower court, Coppi said. In pressing the court to uphold the conviction and jail term, the prosecutor was `’trying to defend an indefensible verdict.”

Earlier, Coppi vowed to deliver a `’win” for Berlusconi with Wednesday’s arguments.

Lawyers said the Court of Cassation’s ruling on Berlusconi’s appeal was expected either late Wednesday or on Thursday.

The ban has been the most worrisome part of the looming punishment to Berlusconi.

Due to a law that automatically trims three years from prison terms in hopes of reducing jail overcrowding, he would be left with only one year to serve. And because of his age — he turns 77 in September — Berlusconi would be eligible to serve that year confined to one of his villas instead of behind bars.

Berlusconi was first convicted with three others in Milan last October of tax fraud in a complex TV rights transaction for his Mediaset network, and sentenced to four years in prison with a five-year ban on public office. A Milan appeals court upheld the lower court’s decision this spring. The Cassation Court offers his final appeal.

In a touch of irony, an attorney for the government asked the high court to also uphold a 10-million euro judgment against the former premier, arguing that the tax fraud damaged the state’s coffers.

Defendants rarely attend hearings at the top court, and Berlusconi was not present.

A dozen police officers were posted in the courtroom, many more than are usually present even during other high-profile cases, to enforce a ban on taking photos and using telephones.

The three-time former premier has no official role in Italy’s uneasy coalition government, but remains influential and was key to the center-right’s close, second-place finish in February elections. He won a seat in Italy’s Senate, which he would have to vacate if the lower courts’ verdicts are confirmed.

Berlusconi has appealed to center-right lawmakers not to yank their support from Letta’s government if the conviction is confirmed.

Letta’s government is already having trouble mustering support in Parliament for a series of measures to help Italy out of recession, although Letta has brushed off any impact by the court decision, saying, `’Italy is more stable than you expect.”

`’The problem is that in the case of a conviction, the dynamic could overwhelm the wishes of individuals,” Stefano Folli, a political analyst for il Sole 24 Ore business daily, wrote Tuesday. `’A convicted Berlusconi would become a martyr for his backers while at the same time being an unacceptable ally for a good part of the center-left.”

The high court is limited in its scope to examining whether errors were made at the trial or earlier appeal. It does not hear new witnesses or receive new evidence.

The court could throw out Berlusconi’s conviction, but such rulings are rare. Another possibility is that it could order a new appeals trial before a different judge. If that happened, the statute of limitations would likely kick in for at least some of the alleged offenses, resulting in a milder sentence if Berlusconi is convicted again, and maybe sparing him the ban on holding public office.

Indicative of the high stakes in this case, Berlusconi for the first time in two decades of court cases has reached beyond his longtime defense team to tap Coppi, a highly reputed appellate lawyer, to argue before the court.

In a separate criminal case yet to come to appeal, Berlusconi faces a possible lifetime ban on holding public office if a lower-court conviction for paying for sex with a minor and using his office to cover it up is upheld. That verdict in June also dealt him a seven-year prison sentence.